C Standard Headers Index

C Standard Headers Index

A grouped index of C standard headers with one-line summaries of what each header provides.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

C Standard Headers Index

Core utilities and diagnostics

<assert.h>: runtime assertions in debug-friendly builds
<errno.h>: thread-local error indicator and error macros
<stddef.h>: size_t, ptrdiff_t, NULL, offsetof, fundamental utility typedefs
<stdint.h>: fixed-width integer types and integer constant macros
<stdbool.h>: Boolean compatibility macros for C
<stdarg.h>: variadic argument handling
<stdlib.h>: allocation, conversions, process control, environment, sort/search helpers

Text, memory, and character handling

<string.h>: raw memory and null-terminated string utilities
<ctype.h>: character classification and case conversion
<uchar.h>: UTF-16 and UTF-32 support types and conversions

I/O and formatting

<stdio.h>: streams, files, formatted I/O, binary I/O, file positioning

Math, time, locale, and signals

<math.h>: transcendental, rounding, and classification functions
<time.h>: wall-clock time, broken-down time, formatting, CPU clock
<locale.h>: locale selection and numeric/string formatting categories
<signal.h>: signal handlers and signal-related macros

Concurrency and low-level facilities

<stdatomic.h>: atomic types and operations
<threads.h>: C11 threads, mutexes, condition variables, thread-specific storage
<stdalign.h>: alignment support macros
<stdnoreturn.h>: noreturn compatibility macros

Practical use

Start here when you remember the facility family but not the exact header.
Then jump to C standard library for the main functions, types, and macros.

Example in practice

#include <vector>
#include <string_view>

int main() {
    std::vector<std::string_view> headers{"<vector>", "<ranges>", "<format>"};
    return static_cast<int>(headers.size());
}